Childhood obesity continues to be a significant public health concern, with the prevalence of obesity among 4- year-old children most recently estimated at 18%. Childhood obesity, once established, is very likely to track into adulthood and prevention of obesity is much more effective than treating obesity once it is established. Effective obesity prevention programs in early childhood are therefore needed, but there have been no effective interventions identified to date that have robust and lasting effects. Childhood obesity is also more common among low-income and minority children, and effective interventions are therefore particularly needed for this group. Head Start is a United States federally-funded preschool program that is free to 3- to 5-year-old children living below the federal poverty line. It serves nearly one million children across the United States each year, and reaches more than half of all eligible children. In order to receive federal funding, Head Start programs are required to adhere to specific performance standards that regulate a substantial amount of programming related to health, nutrition, and physical activity. Despite the broad reach and well-regulated obesity-relevant programming, there has been no evaluation to date of the potential link between Head Start exposure and beneficial effects on obesity prevalence among low-income children. There is no national database that contains anthropometric data of the children enrolled in Head Start, even though every Head Start program is required to maintain these data. The proposed project will utilize existing Head Start anthropometric data from children enrolled in Head Start programs in the state of Michigan over the past 5 years to evaluate changes in children's growth patterns during the academic year (while enrolled in Head Start), compared to the summertime (when not enrolled). The 3 aims of this project are: Aim 1: To test the hypothesis that Head Start exposure will have beneficial effects on the body mass index (BMI) of low-income children during two academic years. We expect that these effects will weaken during the intervening summer, and with less Head Start exposure/dosage. Aim 2: To test the hypothesis that the beneficial effects of Head Start on the BMI of low-income children will be associated with Head Start obesity prevention-specific programming and the nutrition quality of menus. Aim 3: To test the hypothesis that the beneficial effects of Head Start exposure on children's BMI are moderated by environmental/place-based characteristics including neighborhood social disorganization, proximity/distance to parks and green spaces, and grocery store location and type. This project will have broad public health implications in that it will determine if Head Start enrollment is an effective preventive or treatment approach to childhood obesity among low-income children. In addition, it will have an immediate impact by providing feedback to programs around the state of Michigan which may inform programming changes to maximize the effects of individual programs.